The field of the invention is an apparatus and process for converting diluted ethylene in a hydrocarbon stream to heavier hydrocarbons. These heavier hydrocarbons may be used as motor fuels.
Dry gas is the common name for the off-gas stream from a fluid catalytic cracking unit that contains all the gases with boiling points lower than ethane. The off-gas stream is compressed to remove as much of the C3 and C4 gases as possible. Sulfur is also largely absorbed from the off-gas stream in a scrubber that utilizes an amine absorbent. The remaining stream is known as the FCC dry gas. A typical dry gas stream contains 5 to 50 wt-% ethylene, 10 to 20 wt-% ethane, 5 to 20 wt-% hydrogen, 5 to 20 wt-% nitrogen, about 0.1 to about 5.0 wt-% of each carbon monoxide and carbon dioxide and less than 0.01 wt-% hydrogen sulfide and ammonia with the balance being methane.
Currently, the FCC dry gas stream is sent to a burner as fuel gas. An FCC unit that processes 7,949 kiloliters (50,000 barrels) per day will burn about 181,000 kg (200 tons) of dry gas with about 36,000 kg (40 tons) of ethylene as fuel per day. Because a large price difference exists between fuel gas and motor fuel products or pure ethylene it would appear economically advantageous to attempt to recover this ethylene. However, the dry gas stream contains impurities that can poison oligomerization catalyst and is so dilute that ethylene recovery is not economically justified by gas recovery systems.
The oligomerization of concentrated ethylene streams to liquid products is a known technology. However, oligomerization typically involves the use of propylene or butylene particularly from liquefied petroleum gas (LPG) or dehydrogenated feedstocks to make gasoline range olefins. Ethylene is little used as an oligomerization feedstock because of its much lower reactivity.
There is need for utilization of dilute ethylene in refinery streams.